1. Field of the Invention
The present invention relates to a crystallization apparatus which irradiates laser light-to a thin film such as a semiconductor film and to a crystallization method, and more specifically to a laser crystallization apparatus and a laser crystallization method in which melting and crystallization process of the semiconductor film can be observed in a magnified image in real time.
2. Description of the Related Art
A laser crystallization technology has been developed, in which, for example, a high-energy short-pulse laser light is used to melt and crystallize a semiconductor thin film to make a non single-crystal thin film, for example, an amorphous or polycrystal semiconductor film into a crystallized thin film including an area with large crystal grains. This technology is used for crystallization of a non single-crystal semiconductor film used for a thin-film transistor for display devices such as liquid crystal display devices and organic electro luminescence display devices, for example.
Among such laser crystallization technologies, attention is focused on a Phase Modulated Excimer Laser Annealing (PMELA) which irradiates a phase-modulated excimer laser light for crystallization. The PMELA technology forms a homogenized excimer laser light into a laser light having a predetermined light intensity distribution. The laser light is phase-modulated by a phase modulating element, such as a phase shifter, for example, to have an inverse peak light intensity distribution. The laser light is irradiated through a crystallization optical system to a semiconductor film, for example, an amorphous or polycrystal silicon thin film formed on a glass substrate of large-area, such that the semiconductor film is melted and crystallized to form a semiconductor film having large crystal grains. According to the currently developed PMELA technology, an area of a size of about several mm square is melted and crystallized in one irradiation, such that a crystallized silicon thin film with high quality is formed which has relatively uniform and large crystal grains sized from several μm to about 10 μm. Details of which is described, for example, in “Amplitude and Phase Modulated Excimer-Laser Melt-Regrowth Method of Silicon Thin-Films—A New Growth Method of 2-D Position-Controlled Large-Grains-”, published by Kohki Inoue, Mitsuru Nakata, Masakiyo Matsumura, in a thesis journal of Institute of Electronics, Information and Communication Engineers, Vol. J85-C, No. 8, pp. 624-629, 2002.
In the current PMELA technology, excimer laser light power varies from about 5% to 10% in a practical use. However, as compared with the stability of the excimer laser light, a process margin to form a crystallized silicon thin film having a predetermined quality is extremely narrow. Thus, to industrialize the EPMLA technology, the process margin needs to be increased to form the crystallized silicon thin film with a higher and stabilized quality. In response to this, there is a need to observe or measure a change of the silicon thin film through images or the like, in which the silicon thin film is melted in a small area and then crystallized, in real time with a high spatial resolution of several μm, and/or with a high temporal resolution of an order of nanoseconds (hereinafter referred as nsec) immediately after the laser light irradiation.
A method for evaluating crystallinity of a laser-annealed silicon thin film has been disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2001-257176. This method comprises applying observation light to a silicon thin film being crystallized, using a spectrophotometer to subject reflected light to, for example, Raman spectrometry, and thus evaluating the crystallinity of a polycrystal silicon thin film after crystallization.
In an ELA technology which does not perform phase modulation, an experimental example in which thermal characteristics of a melted and crystallized silicon thin film are in-situ-measured by in-situ-measuring means has been reported in M. Hatano, S. Moon, M. Lee, K. Suzuki, and C. Grigoropoulos; J. Applied Physics, Vol. 87, No. 1, pp. 36-43, 2000, “Excimer laser-induced temperature field in melting and resolidification of silicon thin films”. This report regards measurement of thermal characteristics of the melted and crystallized silicon thin film with a high temporal resolution of an order of nsec. More specifically, helium-neon (He—Ne) laser light (wavelengths 633 nm and 1520 nm) as a probe light for observation is applied to melted and crystallized area obliquely from above. Reflected and/or transmitted light from the melted and crystallized area is detected by a high speed responsive indium-gallium-arsenide photodetector and/or by a silicon pn photodiode to measure the thermal characteristics of the silicon thin film.
Furthermore, an in-situ observation method of simultaneously irradiating crystallization laser light and observation light has been disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2002-176009. In the patent, an objective lens with hole is used both for irradiation of the crystallization laser light and for illumination and detection of the observation light. The crystallization laser light is excimer laser light which is not phase-modulated, and is irradiated to a film being processed through the hole provided to the objective lens. In an annealing process, the reflected light by the film being processed is detected though the objective lens with hole to measure in situ changes in reflectance, Raman spectrum and the like on, for example, of a sample surface. That is, crystallinity evaluation is executed from a physical property value of the crystallized area.
Problems in industrializing a mass production line for crystallization utilizing an ELA apparatus used here include improvement in yield of a crystallization process, and stabilization in quality control by monitoring the crystallization process by an operator due to instabilities of irradiating laser light, such as missing of a pulse and/or fluctuation in the intensity, for example. Where an irradiation period of a laser pulse for crystallization is very short, e.g., about 25 to 30 nsec. To solve these problems, there is a need to observe or measure, through images, a changing state or a crystallized area of the silicon thin film in situ and in real time with a high spatial resolution of several μm and with a high temporal resolution of the order of nsec immediately after laser light irradiation, wherein the silicon thin film is melted in a small area in a time period of about ten to several hundreds nsec and then crystallized.
The method in the Jpn. Pat. Appln. KOKAI Publication No. 2001-257176 described above is not suitable for the purpose of observing the crystallization of the silicon thin film through images. The method in the Jpn. Pat. Appln. KOKAI Publication No. 2002-176009 is available for observing through images, however it is not suitable for observing the changing silicon film from melting to crystallization through images with the high temporal resolution and/or high spatial resolution.
The method of M. Hatano et al. has high resolution in time, but is not applicable to an image observation system that satisfies both the high spatial resolution of several μm or less and the high temporal resolution of several nsec at the same time.
The present inventors have found out that, for a higher quality of the crystallized semiconductor thin film, it is desired to install an image observation system, that is, an optical system for observation, in a laser crystallization apparatus, for example, an excimer laser crystallization (ELA) apparatus. In which the semiconductor thin film is changing from a melted state to a crystallized state, and is observed through images with a high spatial resolution of several μm and with a high temporal resolution of the order of nsec, in real time or during laser melting, or the crystallization immediately after melting.
The present inventors have therefore studied incorporating a microscopic observation system capable of image observation into the ELA apparatus in order to enable the in-situ (real-time) observation. To incorporate a microscopic observation optical system into an optical system of the ELA apparatus, it is preferable to use an optical system in which aberration correction has been made simultaneously for excimer laser light for crystallization (ultraviolet light) and illumination light for observation (visible light). The microscopic observation optical system is capable to observe image of a melted and crystallized semiconductor film or a crystallized area, in situ (in real time).
The following problems additionally arise to solve problems described above and to satisfy desire described above. A high resolution (several μm) is preferable in the ELA apparatus, particularly in a projection type phase modulated (PM) ELA apparatus using a phase shifter. It is assumed that lenses practically used in the PMELA apparatus are used at a high light intensity, at high duty and in a large area for production efficiency. More specifically, the laser light intensity is preferably about 1 J/cm2 on a substrate to be crystallized. In order to obtain the high light intensity, the excimer laser light is used with a wide spectral bandwidth (0.5 nm), unlike in an aligner used for large-scale integrated circuit production. Due to the high-energy light, a lens configuration including pasted and laminated lenses such as microscopic lenses for visible light is not preferable in terms of heat resisting properties. Further, the excimer laser light to be used is, for example, krypton fluoride (KrF) or xenon chloride (XeCl), and their wavelengths are 248 nm and 308 nm, respectively. When the wavelengths of these laser lights are taken into account, lens materials that can preferably be used are limited to UV-grade synthetic quarts or calcium fluoride (CaF2), which reduces the freedom in lens designing. Moreover, for example, the correction of aberration, such as chromatic aberration and distortion aberration of the ultraviolet light, has to be made for the lens (group) used in the PMELA apparatus, for example, to transfer a mask pattern of the phase shifter onto the substrate in a reduced or equal size with a high resolution of about several μm.
When the excimer laser light and the microscopic observation visible light are used in this single optical system, the aberration correction has to be made simultaneously in two wavelength areas including the ultraviolet light and the visible light, which is a significantly difficult problem. Even if the chromatic aberration can be corrected, the number of lenses has to be increased, thus increasing the absorption of light by the lenses. This reduces the intensity of the laser light arriving onto the substrate, which goes against the demand to obtain a high light intensity preferred for crystallization.
Another problem is that the crystallization optical system adapted to the excimer laser light with the above-described performance reduces the resolution of the visible light when it transmits the visible light. Specifically, the resolution is proportional to the wavelength of light, so that in the case of the visible light (wavelength: 480 nm to 600 nm) having about double of the wavelength of the excimer laser light (wavelength: 248 nm, 308 nm), for example, a resolution of 2 μm is reduced to about 4 μm in the visible light, which is double the former. As a result, it cannot be obtained a resolution of 1 μm necessary to observe or measure the images of the crystallized area of several μm.
That is, the optical system that can accommodate such a demand need to be stably used for at least two different wavelengths: the excimer laser light for crystallization (e.g., wavelength 248 nm) having a high light intensity (e.g., 1 J/cm2 or higher on the substrate), a large irradiation area (e.g., 5.5 mm2 or larger) and high duty (e.g., a laser operating frequency of 100 Hz or higher), and the illumination light for observation being, for example, the visible light (e.g., wavelengths ranging from 480 nm to 650 nm).
As such an example, a high resolution UV laser condenser lens for microscopes, for example, model KVH20-8, Showa Optronics, is commercially available, in which chromatic aberration is corrected simultaneously in wavelength ranges of both the ultraviolet light and visible light. The condenser lens is designed to use one optical system to process a small area (e.g., 0.5 mm2 or less) with the excimer laser light and to observe with the visible light. The processing of the small area by the optical lens is intended to, for example, cut a part of wiring in an integrated circuit by irradiating the laser light. The lens sufficiently satisfies the desire with the spatial resolution (1 μm), but is not capable of an operation with the above-described high light intensity, large irradiation area and high duty.
Furthermore, an observation needs to be made with a high temporal resolution of an extremely short-time (nsec) to observe in real time through the images of the semiconductor film provided on the substrate in a melting and crystallizing state. It requires a high-luminance illumination light source for observation conforming to the short-time observation. If the visible light is applied as such illumination light for observation through a large number of optical lenses, this causes not only a problem of loss in amount of light but also a problem of an adverse effect on the imaging performance of the original ultraviolet light.
An object of the present invention is to provide a laser crystallization apparatus and a laser crystallization method in which images of an area of several μm on the semiconductor thin film melting and crystallizing in several hundreds nsec can be observed or measured in real time or immediately after that with a high spatial resolution of several μm or less and with a high temporal resolution.